Optical filters have been designed that attenuate sufficiently at a particular wavelength thus leaving the rest of the visible spectrum open. Since the use of filters greater than optical density-2 (OD-2) limits the performance of pilot/operator in bright sunlight conditions, it follows that the use of OD-2 filters in a low ambient light scenario would likewise limit the performance for non-instrumental surveillance at the latter conditions. Bright light in the visible region (e.g., wavelength from about 397 micrometers or microns to about 723 micrometers or microns) and to the infrared region to the limit of perception of human eye (which is about 100 microns or 100 micrometers), emitting from a point source relates to radiation of a different nature than that which is emitting from a wide spectrum electromagnetic radiation source extending in many directions. Radiation from a point source such as radiation from a frequency agile laser is the subject matter which is covered in greater detail hereinbelow.
Laser radiation particularly in the visible region has always presented a potential hazard to the human eye. More recently this potential has been under exploitation by military organizations both foreign and domestic. The human eye focuses light in the visible region which greatly increases the intensity of light on the retina from that striking the cornea and the lens.
Certain laser beams are 10,000 times brighter than the sun's rays, and much more hazardous. When a beam of laser radiation is absorbed by living tissue, the extent of damage caused is dependent on several things: the energy level of the radiation, the type of tissue irradiated, and the wavelength of the laser radiation, and the time of exposure to the radiation.
Intense laser-energy when absorbed by the body is converted into heat. This heat coagulates the protein in the body's tissues (in a similar manner when boiling water coagulates egg albumin) and destroys the cells.
The human eye is the most vulnerable tissue to all types of laser radiation. The tissue in the retina (that portion of the eye upon which the light or image is focused, and specifically the fovea of the retina) is particularly susceptible to damage because the lens of the eyeball concentrates and focuses the laser beam on the fovea of the retina.
The interaction of a laser beam with eyeball tissue as received by the cornea and focused by the lens on the fovea of the retina is described as follows:
1. As the laser beam impinges on the eyeball, part of the beam is prevented from entering the eye by the iris, a colored disc behind the cornea (the outwardly convex transparent membrane forming part of the anterior outer coat of the eye); the iris acts like an automatic photographic shutter and constricts when high-intensity light impinges on the eye;
2. The shutter action of the iris prevents part of the light from reaching the retina;
3. The part of light which reaches the retina also affects a thin, dark-brown choroid membrane containing arteries, veins, and pigment cells which surrounds the retina. This membrane being dark colored can easily absorb this harmful radiation; hence, it must be protected;
4. The laser beam is converged and focused on the fovea of the retina by the lens; and,
5. As projected from laser-welding principles we know that the focus is the hottest point, therefore, the laser-energy density at the fovea (which is the focal point) is about 10.sup.4 to 10.sup.6 times more concentrated than that received by the cornea and the lens.
As concluded from the above described interactions, it is recognized why eye damage can occur due to this magnitude of light concentration.
Since the laser when used as a weapon against the human eye, eyes behind optics, and optics themselves is considered a threat to U.S. Forces, scientists have looked hard at countermeasure techniques. Protective goggles or glasses have been the only real product of research efforts. The bulk of the work has been in the types of filters to go in this eyewear.
Filters have been designed that attenuate sufficiently at a particular wavelength thus leaving the rest of the visible spectrum open. This is a real advantage to the pilots or operators in performing their tasks. With the advent of frequency agile lasers however, it has become necessary to filter over the entire visible region. This presents a real handicap to the operators in performing their tasks particularly in a low ambient light scenario.
Designing a filter in the form of goggles which filter over the entire visible region is not a practical approach. Another name for goggles which filter the complete visible spectrum is blindfolds. Thus, the limitation to an operator's performance is recognized, and it follows that instrument flight conditions would then be required; however, the ability to locate and pinpoint a source of light would be lost. Since OD-2 goggles pass 1/100 of the light that strikes them which means that the agile laser threat could increase laser output by 100 times and thus negate the filters. Finally, the goggles allow for exposure of both eyes simultaneously. It follows that another approach is desired since complete protection of both eyes from a first laser pulse does not appear possible. Therefore, scientists had at this point in time shifted their emphasis from protection from the first laser pulse to a system designed towards providing protection from the second and subsequent laser pulses. The first pulse protection is not as essential to protect against since it appears that an operator will not be looking directly at the laser source when the first pulse arrives or that the first pulse will be off target but still detectable. This scheme also protects the operator from instinctively looking towards the source when the next pulse arrives.
The invention described hereinbelow takes another approach to solving the problem of providing eye/optics protection. This approach provides protection from laser pulses, including the first one, while also avoiding the risk of damage to both eyes simultaneously. The operator will also suffer no degradation of his performance due to insufficient light and should be able to pinpoint the hostile laser source location.
An object of this invention is to provide a device removeably, attachable to an operator's headgear which provides alternating clear and nearly opaque sections of a rotating shutter to provide eye protection while retaining visual acuity for operator.
A further object of this invention is to provide a rotating shutter having a predetermined ratio of open sectors to opaque sectors, predetermined sector size, and predetermined variable rotational speed to optimize maximum eye protection while retaining maximum visual acuity for an operator.
Still a further object of this invention is to provide a rotating shutter having the predetermined ratio number of open sectors to opaque sectors and predetermined rotational speed so that the rate of light reaching the retina of the operator achieves the critical flicker fusion frequency at low level illumination intensity of 5 to 6 flashes per second and at high level illumination intensity of up to 60 per second.